The actress - who will return to the BBC series as Rose Tyler in the 50th anniversary special - said that Capaldi's casting "makes absolute sense".
Speaking to Flicks and the City about Capaldi's casting, Piper revealed: "I think it's brilliant. I've always really loved him and admired him. I think he's a brilliant actor, I think he's both absolutely terrifying and credible with comedy.
"I really like the fact that he's a bit older and I think it will open a whole new set of fans. I think it makes absolute sense. It's a really wise choice."
Piper described the upcoming special 'The Day of the Doctor' as a "gift to the fans".
The 31-year-old explained: "I'm just looking forward to the fans being able to finally see it, because there is relentless questioning about what is going to happen and I can't say anything.
"It will be nice to be rid of the burden of that enormous secret. It's like a gift to the fans really so all in all I think it's pretty exciting."
Piper - who last appeared on Doctor Who in David Tennant's final episode 'The End of Time' - admitted that she had some nerves about returning.
"I never feel that anxious about returning because I'm friends with a lot of people and I've built some really good relationships. I always know it's going to be beautifully written," she added.
"I think you're anxious on the first day in terms of getting back into the character and whether you've still got it there. Do you know what I mean? Otherwise, it's pretty seamless and enjoyable."
Doctor Who's 50th anniversary special - also starring Matt Smith, David Tennant, Jenna Coleman and John Hurt - will air worldwide on November 23.